in vitro comparative study of compressive strength of different types of composite resins in different periods of time

to evaluate the ultimate compressive strength of five composite resins after 1 hour, 24 hours, 7 days and 1 month. twenty four cylindrical 4 mm×8mm specimens of each commonly used composite resins in posterior region (nulite-f , z250 , p60, biscore, tetric ceram hb) were prepared. each group of composite resins were divided into four time groups of 1 h, 24 h, 7 days and 1 month. all of specimens stored in an incubator with 37 °c. after each period of time all of the specimens were tested by zwick/roell z020 (germany) for ultimate compressive strength at cross head speed of 0.5 cm/min. results were analyzed by anova and scheffe test. p60 and z250 had the highest and nulite-f and tetric ceram hb had the lowest compressive strength at all the times. the difference between these two groups was statistically significant (p < 0.05). the results of the biscore was steady among the other groups. it is suggested to use z250 or p60 in posterior restorations instead of the other composite resins tested.